Microorganisms are involved in the manufacture of food and feed products including most dairy products. Thus, bacterial cultures, in particular cultures of bacteria that are generally classified as lactic acid bacteria are essential in the making of all fermented milk products, cheese and butter. Cultures of such bacteria are referred to as starter cultures and they impart specific features to various dairy products by performing a number of functions.
Commercial dairy starter cultures are generally composed of lactic acid and citric acid-fermenting lactic acid bacteria. In the present context, the expression “lactic acid bacteria” designates a group of Gram positive, catalase negative, non-motile, microaerophilic or anaerobic bacteria which ferment sugar with the production of acids including lactic acid as the predominantly produced acid, acetic acid, formic acid and propionic acid. The industrially most useful lactic acid bacteria are found among Lactococcus species, Streptococcus species, Enterococcus species, Lactobacillus species, Leuconostoc species and Pediococcus species.
Commonly used dairy starter culture strains of lactic acid bacteria are generally divided into mesophilic organisms having optimum growth temperatures at about 30° C. and thermophilic organisms having optimum growth temperatures in the range of about 40 to about 45° C. Typical organisms belonging to the mesophilic group include Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Leuconostoc mesenteroides subsp. cremoris, Pediococcus pentosaceus, Lactococcus lactis subsp. lactis biovar. diacetylactis and Lactobacillus casei subsp. casei. Thermophilic lactic acid bacterial species include as examples Streptococcus thermophilus, Enterococcus faecium, Lactobacillus lactis, Lactobacillus helveticus, Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus acidophilus. 
Also the strict anaerobic bacteria belonging to the genus Bifidobacterium including Bifidobacterium bifidum and Bifidobacterium longum are commonly used as dairy starter cultures and are generally included in the group of lactic acid bacteria. Additionally, species of Propionibacterium are used as dairy starter cultures, in particular in the manufacture of cheese.
Additionally, organisms belonging to the Brevibacterium genus are commonly used as food starter cultures.
Another group of microbial starter cultures is fungal cultures, including yeast cultures and cultures of filamentous fungi, which are particularly used in the manufacture of certain types of cheese and beverage. Examples of currently used cultures of fungi include Penicillium roqueforti, Penicillium candidum, Geotrichum candidum, Torula kefir, Saccharomyces kefir and Saccharomyces cerevisiae. 
Presently, commercial starter cultures are commonly distributed as frozen concentrates. Under these conditions, the viability of the cultures is preserved for extended periods of time and the cultures can be inoculated directly into milk without intermediate transfer. Such cultures are generally referred to as direct vat set (DVS)-cultures. Another presentation of commercial DVS-starter cultures is as freeze-dried or lyophilised cultures in the form of a powder. In this form, the starter can be shipped without refrigeration, but storage below freezing temperature is recommended.
Although commercial starters thus are available as cultures, which can be added directly to milk without any intermediate transfer or propagation, it is not uncommon that dairies produce in-house bulk starters at regular intervals depending on the requirement. A “bulk starter” is defined herein as a starter culture propagated at the dairy plant for inoculation into milk. Such bulk starters are generally made by inoculating heat treated milk with a volume of a previous bulk starter or with a freeze-dried or frozen starter culture preparation, followed by incubating the thus inoculated milk under conditions permitting the starter culture strain(s) to propagate for a sufficient period of time to provide a desired cell number. The incubation period is typically in the range of 4 to 24 hours.
However, the preparation of such bulk starter cultures is labour intensive and it occupies much space and equipment, and there is a considerable risk of contamination with spoilage bacteria and/or phages during the step of propagation.
The use of commercial liquid starter cultures in the food and feed manufacturing industry including the dairy industry has been suggested as a useful alternative to the use of commercial frozen and freeze-dried starter cultures. The advantages for the industry by having such liquid starter cultures at its disposal would be several. Thus, it would be highly convenient and much less labour consuming to handle such starter cultures at food and feed manufacturing plants as compared to the use of the conventional frozen or freeze-dried cultures. Thus, when using liquid starter cultures, the inoculation of the material to be inoculated can be made directly e.g. by connecting the container with the liquid culture directly to the process line, thus avoiding the tedious work connected with opening several packagings of culture prior to inoculation. Additionally, it can be avoided to open the process line, as it is required when using frozen or freeze-dried cultures, which reduces the risk of contamination.
However, the use of commercial liquid starter cultures has so far not been feasible or possible, as such cultures, even if the cells of the cultures keep their viability, rapidly loose their metabolic activity such as e.g. their acid-producing (acidification) activity when kept stored even for shorter periods of time. To be commercially useful, liquid starter cultures should preferably retain their metabolic activity for at least 1 week and more, preferably for at least 2-3 weeks. Up till now it has not been possible to provide commercial liquid starter cultures having such a high stability.
It is therefore an important objective of the present invention to provide liquid starter cultures which show a high degree of storage stability in respect of retaining the metabolic activity when kept under cool storage conditions for extended periods of time.